Metallic painting and pearl color painting used for automotive painting and the like contain flake-like aluminum pieces and mica pieces called glitter materials in paint films, and exhibit what is called a metallic effect and a pearl effect. These effects are attributable to the fact that the contribution of glitter materials to the reflection characteristics varies depending on the illumination direction and the observation direction. Devices for evaluating such metallic painting and pearl color painting have been known conventionally.
A coating film containing such a glitter material is evaluated by calculating color information of a subject (for example, tristimulus values) as the predetermined optical characteristics based on the spectroscopic result obtained with multi-angle geometry. Such an evaluation is performed, for example, using a multi-angle colorimeter disclosed in Patent Literature 1. The multi-angle geometry includes geometry of multidirectional illumination/unidirectional light reception in which light beams are radiated to a measurement surface of a subject from a plurality of different directions and received in a single direction, and geometry of unidirectional illumination/multidirectional light reception in which light beams are radiated to a measurement surface of a subject from a single direction and received in a plurality of different directions. The geometry is defined in, for example, DIN-6175-2 of DIN standards (German industrial standards) and the like.
Further, a coating film containing a glitter material is evaluated by calculating brilliance as the predetermined optical characteristics based on an image of a subject obtained with multi-angle geometry. Such an evaluation is performed, for example, using a brilliance evaluation device disclosed in Patent Literature 2. The brilliance is quantified from the image of the subject using the number of bright points due to glitter material reflection or the distribution of the size of the bright points.
In order to radiate light (or receive (observe) light) from a plurality of directions with the multi-angle geometry, an optical system and a light source (or a light receiving portion) are required for each angle, which makes the configuration complicated. Furthermore, since it is necessary to change the illumination direction (or the light receiving direction (observation direction)), the measurement time is long. For this reason, for example, a multi-angle colorimeter of Patent Literature 3 has been proposed. The multi-angle colorimeter disclosed in Patent Literature 3 includes: an annular first reflecting mirror (for example, a toroidal mirror) having a center axis on a sample surface; a second reflecting mirror (for example, a conical mirror) provided near a focal circumference including a group of focal points of the first reflecting mirror; a relay optical system for forming an image of the focal circumference around the central axis as an optical axis; an aperture plate provided on an image forming surface of the relay optical system and having one or more sample light apertures on an aperture circumference that coincides with the image of the focal circumference; and a light receiving sensor provided behind the sample light aperture to receive reflected light reflected by the sample. Parallel light fluxes emitted from the sample surface toward the first reflecting mirror are reflected by the first reflecting mirror, converged on the focal circumference, reflected by the second reflecting mirror provided near the focal circumference to enter the relay optical system, and converged on the aperture circumference to pass through the sample light aperture and enter the light receiving sensor.
The multi-angle colorimeter disclosed in Patent Literature 3 is advantageous because the optical system is simplified by the above configuration. However, it is necessary to make an adjustment so that the focal plane of the first reflecting mirror (toroidal mirror) coincides with the focal plane of the relay optical system. In this adjustment, since light beams from the respective light receiving directions (observation directions) are reflected by the single first reflecting mirror, the traveling directions of the light beams from the respective light receiving directions (observation directions) must be adjusted at the same time by adjusting the position and posture of the single first reflecting mirror. Further, since the position and posture of the second reflecting mirror (conical mirror) also relate to the adjustment, it is difficult to assemble and adjust these components. Generally, the toroidal mirror and the conical mirror are expensive due to their high cost of production.